Oriented strand board (OSB), as described in the Structural Board Association (SBA) bulletin of November 1993, is composed of longitudinally extending strands of hardwood, e.g. aspen, poplar, southern yellow pine, formed by slicing logs in the direction of the grain. After slicing, the strands are dried, blended with wax and a waterproof binder, i.e. adhesive, usually a phenolic resin, and formed into a loose mat of several layers and the mat is hot pressed to bind the wood strands together and form a rigid, dense panel suitable for structural use. A typical commercial OSB structural panel comprises a core of cross-aligned layers bonded to and covered on both sides by face layers in longitudinal alignment. This configuration enhances the strength and stiffness properties of the panel. Such OSB structural elements are increasingly in demand as noted in Panel World, November 1993, pages 9-12, and it is further noted therein that the production of fire retardant OSB has not yet been perfected.
The fire retardant OSB structural element of the present invention is a hot pressed oriented strand board panel which comprises a core of adhesive coated oriented strands of wood bonded together, the core being joined in the course of hot pressing to a pair of covering outer layers of adhesive coated oriented layers of adhesive coated oriented strands of wood. Each said covering outer layer has incorporated therein from 1 to 10% by weight of particles of expandable graphite, and the core being free of expandable graphite, with the volume of the core being from 40% to 60% of the volume of the structural element so that upon exposure of the structural element to external flame, the expandable graphite particles in the outer covering layers will undergo intumescence and increase in volume and establish adjacent to the core a covering adherent layer of vermiform expanded graphite which forms a flame barrier for the underlying core layer.